gdm: Remove separate greeter universal access icon

The idea behind always showing the icon on the login screen is that
the users' needs aren't known at that point. However we can achieve
the same behavior by including the 'always-show-universal-access-status'
key in GDM's presets, so drop the special-case.

https://bugzilla.gnome.org/show_bug.cgi?id=788943